1. Field of the Invention
The invention relates generally to the field of quartz crystals used as highly stable frequency standards (such as clocks). More particularly, the invention relates to techniques for profiling or characterizing the frequency output of crystal-based oscillators with reduced deviations in frequency due to environmental effects.
2. Background Art
Timing of operation of electronic devices, particularly digital devices, requires an accurate, frequency stable clock signal. Many such electronic devices are subjected to variations in ambient temperature during their operation. As is well known in the art, changes in ambient temperature affect the frequency of a typical crystal.
While quartz oscillators are considerably more stable when compared to other types of oscillators, their frequency output is known to exhibit some drift under rapid temperature variations. The effect of stresses on quartz crystals is well known and exploited in the design of quartz based stress and pressure sensors. Due to the low thermal conductivity and the anisotropic properties of quartz, heating and cooling crystals is known to cause stresses in the crystal, which affects the frequency. See, Bottom, Virgile E. Introduction to Quartz Crystal Unit Design, New York: D. Van Nostrand, 1982. For this reason, it is generally not recommended to subject quartz crystals to rapid temperature gradients.
In conventional applications the frequency deviations of quartz crystals due to temperature are profiled or characterized during manufacture and compensated for in real time. Rapid temperature fluctuations in the crystal's environment and changing temperature rates, also referred to as temperature gradients, cause the crystal frequency to deviate from the characterization. Though the varying temperature rates may last for a brief period, their effects can last for long periods of time, causing measurement errors.
In order to achieve greater frequency stability, several methods have been proposed to account for these deviations. One approach is to place the crystal in a temperature controlled chamber, which will keep the crystal at a constant temperature and prevent any deviation in frequency. See, for example, U.S. Pat. Nos. 5,917,272, 5,729,181, 5,180,942, 4,586,006 and 3,619,806. Another approach taken to compensate for the deviations in frequency due to temperature is to use a voltage-controlled oscillator of which the frequency can be adjusted by changing the voltage at the control input. In these designs, the temperature at the crystal is measured and used to digitally compute a correction voltage to be applied to the voltage-controlled oscillator. See, for example, U.S. Pat. Nos. 5,668,506, 5,473,289, 5,214,668, 5,170,136, 5,081,431, 4,922,212, 4,746,879, 4,427,952 and 4,380,745.
One problem with using a temperature sensor and measuring the temperature outside the crystal is that there is a time lag between the actual crystal temperature (at the quartz plate) and the outside where the temperature is measured. This causes the oscillators to be slow in responding to a change in temperature, introducing errors. A proposed solution to this problem is to have the crystal oscillate in two modes simultaneously, where one of the two modes is temperature sensitive while the second mode is relatively stable with temperature. The temperature sensitive mode is used to obtain the temperature at the crystal itself and then used to compensate for minor deviations with temperature in the stable mode. See, for example, U.S. Pat. Nos. 5,525,936 and 4,079,280. In spite of the very accurate measurement of temperature in these designs, high temperature gradients in the environment still introduce errors. Modern crystals are also cut at special angles, such as the SC cut, in an attempt to minimize frequency deviation due to temperature.
Another method that can be used to minimize problems due to fluctuating temperature rates is to place the crystal in a temperature controlled chamber. See, for example, U.S. Pat. No. 6,606,009 (assigned to the present assignee). However, this option entails higher power consumption, which can be a disadvantage in certain applications. Thus a need remains for improved techniques to account for and minimize frequency deviation in crystal-based oscillators due to environmental variations.